Donald Trump, the Republican presidential candidate, has called again for tougher U.S. border security following Tuesday's deadly attacks on the Brussels airport, saying "we have to be very vigilant and careful about who we allow into our country."

Trump's comments, in an interview on NBC's "Today" program, came a day after he expressed skepticism about the U.S. role in the North Atlantic Treaty Organization and said the United States should significantly cut spending on the defense alliance.

"As president ... I would be very, very tough on the borders, and I would be not allowing certain people to come into this country without absolute perfect documentation," he told NBC.

Republican rival John Kasich, the governor of Ohio, struck a different tone, pledging to "redouble our efforts with our allies" and saying the United States "must strengthen our alliances" in the face of acts of terror.

U.S. Senator Ted Cruz, who is running second to Trump in the Republican delegate count, called the blasts in Belgium "the latest in a string of coordinated attacks by radical Islamic terrorists."

"Radical Islam is at war with us," he said in a statement posted online.

Adam Schiff, the top Democrat on the U.S. House Intelligence Committee, said the Brussels attacks that killed at least 26 people at the airport and a rush-hour metro train, "bear all the hallmarks of an ISIS-inspired, or ISIS-coordinated attack," using an acronym for Islamic State.

The attacks will likely revive national security as a key issue in the 2016 race for the White House, at least for now.

Trump looks to take another step toward winning the Republican presidential nomination in contests in Arizona and Utah on Tuesday, aiming to deal another setback to the party establishment's flagging stop-Trump movement.

The billionaire businessman has rolled up a big lead in convention delegates who will pick the Republican nominee, defying weeks of attacks from members of the party establishment worried he will lead the Republicans to defeat in the Nov. 8 presidential election.

In Arizona, one of the U.S. states that borders Mexico, Trump's hardline immigration message is popular and he leads in polls, while in Utah Trump lags in polls behind Cruz, a U.S. senator from Texas.